This invention is directed to nonwoven webs having improved softness and barrier to liquid penetration. The invention is also directed to a method of preparing the nonwoven webs.
Soft nonwoven webs of entangled fibers or filaments are known, for instance, from U.S. Pat. No. 4,443,513, issued to Meitner et al. Soft nonwoven webs, and laminates of them, are useful in applications where softness and bulk are desired attributes including wipers, garments, surgical drapes, diapers and the like. The nonwoven webs may be meltblown thermoplastic fiber webs as disclosed in U.S. Pat. No. 4,307,143, issued to Meitner. These meltblown webs can be produced by meltblowing polypropylene or another thermoplastic through a die having a row of apertures, and impinging heated air at the die exit to draw the filaments, forming microfibers which are then quenched and collected on a moving wire. The nonwoven web may also be a laminate including a meltblown web, for instance a laminate including two spunbond thermoplastic webs with a meltblown web between them. Spunbond/meltblown/spunbond web laminates are disclosed in U.S. Pat. No. 4,041,203, issued to Brock et al. Purely spunbond webs, i.e. not laminated to a meltblown web, are also known in the art.
The above-identified U.S. Pat. No. 4,443,513 discloses that the softness, bulk and drapability of nonwoven webs can be improved by proper selection of interfilament bond patterns in a nonwoven web, and/or by controlled stretching of the webs. The controlled stretching takes place under cool or room temperature. The stretching is limited to the elongation required to break the web. The disclosed thermoplastic non-elastic webs can be stretched to about 1.2-1.4 times their original length.
While spunbond webs promote strength, meltblown webs are known to provide barrier to penetration by liquids, including liquids under pressure. Thus, meltblown webs, or laminates of spunbond and meltblown webs, are generally employed in applications requiring both softness and liquid barrier. There is always a need or desire for fabrics having improved liquid barrier, particularly in medical applications involving hospital and surgical gowns, and the like.
xe2x80x9cNonwoven webxe2x80x9d means a web having a structure of individual fibers or threads which are interlaid, but not in an identifiable, repeating manner. Nonwoven webs have been, in the past, formed by a variety of processes such as, for example, melt-blowing processes, spunbonding processes and bonded carded web processes.
xe2x80x9cAutogenous bondingxe2x80x9d means bonding provided by fusion and/or self-adhesion of fibers and/or filaments without an applied external adhesive or bonding agent. Autogenous bonding may be provided by contact between fibers and/or filaments while at least a portion of the fibers and/or filaments are semi-molten or tacky. Autogenous bonding may also be provided by blending a tackifying resin with the thermoplastic polymers used to form the fibers and/or filaments. Fibers and/or filaments formed from such a blend can be adapted to self-bond with or without the application of pressure and/or heat. Solvents may also be used to cause fusion of fibers and filaments which remains after the solvent is removed.
xe2x80x9cMeltblown fibersxe2x80x9d means fibers formed by extruding a molten thermoplastic material through a plurality of fine, usually circular, die capillaries as molten threads or filaments into a high velocity gas (e.g. air) stream which attenuates the filaments of molten thermoplastic material to reduce their diameter, possibly to microfiber diameter. Thereafter, the meltblown fibers are carried by the high velocity gas stream and are deposited on a collecting surface to form a web of randomly disbursed meltblown fibers. Such a process is disclosed, for example, in U.S. Pat. No. 3,849,241 to Butin, the disclosure of which is hereby incorporated by reference.
xe2x80x9cSpunbonded fibersxe2x80x9d refers to small diameter fibers which are formed by extruding a molten thermoplastic material as filaments from a plurality of fine, usually circular, capillaries of a spinnerette with the diameter of the extruded filaments then being rapidly reduced as by, for example, eductive drawing or other well-known spunbonding mechanisms. The production of spunbonded nonwoven webs is illustrated in patents such as, for example, in U.S. Pat. No. 3,802,817 to Matsuki et al. and U.S. Pat. No. 5,382,400 to Pike et al. The disclosures of these patents are hereby incorporated by reference.
xe2x80x9cPolymerxe2x80x9d generally includes, but is not limited to, homopolymers, copolymers, such as, for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term xe2x80x9cpolymerxe2x80x9d shall include all possible geometrical configurations of the material. These configurations include, but are not limited to, isotactic, syndiotactic and random symmetries.
xe2x80x9cBicomponent fibersxe2x80x9d refers to fibers which have been formed from at least two polymers extruded from separate extruders but spun together to form one fiber. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the bicomponent fibers and extend continuously along the length of the bicomponent fibers. The configuration of such a bicomponent fiber may be, for example, a sheath/core arrangement wherein one polymer is surrounded by another or may be a side-by-side arrangement or an xe2x80x9cislands-in-the-seaxe2x80x9d arrangement. Bicomponent fibers are taught in U.S. Pat. No. 5,108,820 to Kaneko et al., U.S. Pat. No. 5,336,552 to Strack et al., and European Patent 0586924. For two component fibers, the polymers may be present in ratios of 75/25, 50/50, 2/75 or any other desired ratios.
xe2x80x9cBiconstituent fibersxe2x80x9d refers to fibers which have been formed from at least two polymers extruded from the same extruder as a blend. The term xe2x80x9cblendxe2x80x9d is defined below. Biconstituent fibers do not have the various polymer components arranged in relatively constantly positioned distinct zones across the cross-sectional area of the fiber and the various polymers are usually not continuous along the entire length of the fiber, instead usually forming fibrils which start and end at random. Biconstituent fibers are sometimes also referred to as multiconstituent fibers. Fibers of this general type are discussed in, for example, U.S. Pat. No. 5,108,827 to Gessner. Bicomponent and biconstituent fibers are also discussed in the textbook Polymer Blends and Composites by John A. Manson and Leslie H. Sperling, copyright 1976 by Plenum Press, a division of Plenum Publishing Corporation, New York, N.Y., IBSN 0-306-30831-2, at pages 273 through 277.
xe2x80x9cBlendxe2x80x9d means a mixture of two or more polymers while the term xe2x80x9calloyxe2x80x9d means a sub-class of blends wherein the components are immiscible but have been compatibilized. xe2x80x9cMiscibilityxe2x80x9d and xe2x80x9cimmiscibilityxe2x80x9d are defined as blends having negative and positive values, respectively, for the free energy of mixing. Further, xe2x80x9ccompatibilizationxe2x80x9d is defined as the process of modifying the interfacial properties of an immiscible polymer blend in order to make an alloy.
xe2x80x9cMicrofibersxe2x80x9d means small diameter fibers having an average diameter not greater than about 100 microns, for example, having an average diameter of from about 0.5 microns to about 50 microns, or more particularly, an average diameter of from about 4 microns to about 40 microns.
xe2x80x9cNonwoven web bond patternxe2x80x9d is a pattern of interfilament bonding in the nonwoven web which is imparted during manufacture of the nonwoven web.
xe2x80x9cInterfiber bondingxe2x80x9d means bonding produced by entanglement between individual fibers to form a coherent web structure without the use of thermal bonding. This fiber entangling is inherent in the meltblown processes but may be generated or increased by processes such as, for example, hydraulic entangling or needle punching. Alternatively and/or additionally, a bonding agent can be utilized to increase the desired bonding and to maintain structural coherency of a fibrous web. For example, powdered bonding agents and chemical solvent bonding may be used.
xe2x80x9cLiquid pressure resistancexe2x80x9d (also known as xe2x80x9chydroheadxe2x80x9d) refers to the ability of a composition or film made therefrom to withstand the application of a load of liquid without leakage. The liquid pressure resistance of a film depends on the thickness of the film, the material composition of the film, how the film is made and processed, the surrounding environment, and the method of testing. Methods of testing the liquid pressure resistance of a film or material include without limitation the Hydrostatic Pressure Test described in Method 5514 of Federal Test Methods Standard No. 191A, which is equivalent to AATCC Test Method 127-89 and INDA Test Method 80.4-92.
xe2x80x9cConsisting essentially ofxe2x80x9d does not exclude the presence of additional materials which do not significantly affect the desired characteristics of a given composition or product. Exemplary materials of this sort would include, without limitation, pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters, particulates and materials added to enhance processability of the composition.
The present invention relates to a nonwoven web having excellent softness and improved liquid barrier (hydrohead resistance), and laminates including the nonwoven web. The invention also includes a process for making the nonwoven web.
The nonwoven web of the invention includes a meltblown web. The nonwoven web may be a single layer (meltblown) nonwoven web, or may be a laminate of a meltblown web to one or more additional nonwoven webs. The additional nonwoven webs may be additional meltblown webs, spunbond webs, bonded carded webs, or any web or webs that can be used advantageously in combination with the meltblown web. The meltblown web, with or without another nonwoven web, can also be combined with a plastic film, foam, or other entity.
The nonwoven web is heated and stretched between two or more pairs of draw rollers. It has been discovered that the draw rollers can be configured so that the stretching causes the nonwoven web to have improved hydrohead strength as well as softness. Specifically, the drawing distance (i.e. the distance over which the nonwoven web is drawn between two adjacent pairs of draw rollers) is maintained at less than about 35 inches for a single-stage drawing process including only two pairs of adjacent draw rollers, while the heated nonwoven web is stretched by about 1% to about 35% of its initial length. Ideally, the drawing distance is maintained at less than 10 inches while the heated nonwoven web is stretched by about 1% to about 35% of its initial length.
In another embodiment, the nonwoven web is drawn in multiple stages using three or more pairs of draw rollers. When multiple stages are employed, the total drawing distance (i.e., the sum of the distance between adjacent pairs of draw rollers) is maintained at less than about 35 inches while the nonwoven web is stretched by about 1% to about 35% of its original length. One major advantage of using a multi-stage drawing process (as opposed to a single-stage draw) is that the product has a lower basis weight following the drawing, allowing a comparable product to be obtained having greater area with adequate liquid barrier and softness.
Other advantages of short distance drawing, whether single-stage or multi-stage, are reduced neck-in of the nonwoven webs during stretching, increased cup crush strength, and increased tensile strength.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying examples and drawings. The detailed description, examples and drawings are intended to be illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof.